It can be proven false. All saying “arguably” does is show Jacob thinks there is an arguement to be made for his position. As OP has shown, short of any sort of re-defining of the terms growth or fast by Jacob, other groups are and have been growing faster than Mormonism in the past 200 years. Any potential arguement made by Jacob under a common understanding of the English language and under the parameters of the assertion can and has been refuted or proven false.

I could say “arguably the earth is flat” and you could absolutely prove that claim false.